Technical Abstract:
Field-grown grain sorghum [Sorghum bicolor (L.) Moench] was exposed to CO2 concentrations enriched to 200 umol/mol above ambient (ca. 370 umol/mol) using free-air CO2 enrichment (FACE) apparatus. Half of each main CO2 plot received ample water (Wet treatment), while the other half received a deficit amount (Dry treatment), thereby exposing the plants to severe drought. Measurements were made of net radiation, canopy temperature, air temperature, soil heat flux, and wind speed. Energy balance components, including evapotranspiration, were calculated from the data. Under the Dry treatment, changes in apparent midday canopy temperature due to FACE were inconsistent and depended on time since last irrigation. Furthermore, the Dry treatment greatly reduced leaf growth, while elevated CO2 increased growth, both of which affected the degree to which hot soil temperatures and the presence or absence of shadows confounded the apparent canopy temperature readings. In contrast, under well-watered conditions, which had a full canopy, elevated CO2 increased midday canopy temperatures consistently, about 1.3 plus or minus 0.1 degree C on average. Also under the Wet treatment, elevated CO2 reduced evapotranspiration about 10 plus or minus 1 %.